Improve text of resolver warning

[StevenMaude]

Improve the text of the resolver warning to clarify the warning's cause and fix the grammar.

Contributor checklist

• Assure PR title is short, clear, and good to be included in the user-oriented changelog

Maintainer checklist

• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[atugushev]

Thank you!

[StevenMaude]

(As my edits made the strings longer than Black's line length, this change is formatted using black --preview to use Black's experimental string processing feature. This is why the trailing spaces changed to leading spaces: because of this autoformatting.)